1. Field of the invention
The present invention relates to cholesteric liquid crystal composition having arbitrary color-forming temperature range from -5.degree. C. to +45.degree. C.; a color-forming liquid crystal composite products such as film, fiber, coating material, ink or the like using said compositions; a method for protecting printed or pictured liquid crystals; and a color-forming liquid crystal picture laminated products whose visible pictures on a same surface can vary depending upon temperature-variation.
2. Prior art
As for liquid crystals which will provide reversible color change due to temperature-change, steroidal cholesteric liquid crystals (e.g. cholesteryl fatty esters) and color-forming chiral pneumatic liquid crystals are known. In addition, as for steroidal cholesteric liquid crystals, they can be used as color-forming liquid crystals. Cholesteryl esters as a single substance are solid at ordinary temperatures except for few exceptions, and generally when they have lower or higher melting points, they can be liquid crystal only at about 50.degree. C., or at about 200.degree. C. respectively. Therefore, some kinds of these esters are selected, and then they are melt-mixed at a specified mixing ratio thereof for obtaining a lowered solidifying point to allow the mixtures to form color in a predetermined temperature range.
As for even such mixed cholesteric liquid crystals carefully prepared, if they are once left below the color-forming temperatures, they are easily solid-crystallized and can not be again liquid crystal even when heated up to said coloring temperatures. As far as mixed cholesteric liquid crystals are concerned, such phenomena can be considered to occur because of their rapid crystallization based on the same principles as in those of metamorphic differentiation or recrystallization, with the components or crystals thereof changed with no fusion as dealt with in lithology, etc.
As an effective means for preventing such crystallization, microencapsulization can be given. Microencapsulization can hold liquid crystals in a condition which permits no solid-crystallization thereof under ordinarily conditions by the pulverizing of the solid-crystallizable liquid crystals. (Also in other liquid substances, when they are fine particles, they are left liquid even at the temperatures below their solidifying points.).
Because of such reasons, any cholesteric liquid crystal of steroid family which is used about at room temperatures has been microencapsulized.
When microencapsulized liquid crystals, however, are applied for printing, usually they are given serious restrictions due to easy breaks in the capsules caused by printing pressure.
In order to make microencapsulized liquid crystals serviceable in practice, they are needed to be dispersed into liquid binders, however, they are used only in an amount up to about 30 wt % relative to the total amount of dry residue after the removal of solvent therein. Therefore, the face on which the liquid crystal is applied will have lowered clarity resulted from irregular reflection as well as coloring due to the contamination by the crusts of capsules used in the microencapsulization process. Accordingly, a jewel like beautiful coloring due to the great refractive index of light (about 20 times as strong as that of a rock crystal) being the characteristic of cholesteric liquid crystals is lost to have reduced decorative value.
On the other hand, the color-forming chiral pneumatic liquid crystals are stable than the cholesteric mixed liquid crystals of steroid family, however, the use of the former liquid crystals are limited because of their extremely narrow color-forming ranges.
Cholesteryl fatty ester liquid crystals which can be stably storaged for more than six months at room temperatures were disclosed by Japanese Patent Laid-open No. 48559/1985 (Applicant: U.S. RPR Inc.). The cholesteric liquid crystals are used for coloration at a temperature not less than 20.degree. C. (The lowest color-forming temperature of the exemplary liquid crystals of the examples of said Patent was 37.degree. C.), however, their color-forming temperature ranges are so extremely expanded to higher side of temperature that they can not be available for the most commonly used applications which require color-formations corresponding to the temperature-variation in the vicinity of room temperatures.
The present inventors, therefore, for improving these problems, have made various investigations and as a result, have developed cholesteric liquid crystal compositions which are stable for a long period of time and; therefore, require no microencapsulization (Japanese Patent Application No.155186/1989). Accordingly, conventional microencapsulizations have been no more needed, moreover, they are stable liquid crystal compositions for a long period and are available for the processing of fibers, dispersion coatings, etc. Nevertheless, the surface of the liquid crystal picture patterns which are obtained by means of the direct printing or picturing of the base boards made of plastics, paper, metals, etc., being different from microencapsulization liquid crystal or solid matrix agent dispersing liquid crystal, are bare-mesomorphic; therefore, said surfaces are of high viscous liquid, and can not be touched by our fingers without using protecting coating, moreover easily contaminated owing to environmental conditions. The practical use thereof in said conditions are rare, and even if protecting coatings are applied thereto, such coatings necessarily have no adverse effect on the printed or pictured liquid crystal picture patterns, also adhere closely to the patterns without containing air bubbles, as well as are transparent and tough.
Conventionally, a liquid crystal thermography and liquid crystal thermometer, etc. for practical use, which provide coloration-change according to temperature-variation usually have a construction comprising a sheet of film-like dry solid sandwiched between a surface-protecting film and a black backing film using, and said dry solid is made from a binder having an microencapsulized liquid crystal dispersed. A particular one point, therefore, on the film can display its color-forming change corresponding specified one type of liquid crystal. Consequently, if the color-forming range of the film is extended, the sensitivity to a temperature-change becomes poor and resulting in decreased sensing density not to response to small temperature-change. Contrary, if the film is allowed to be responsible to a small temperature-change, its sensing density will become increased to have disadvantageously narrow color-forming ranges. Therefore, conventional liquid crystal thermometers unavoidably have liquid crystals of different color-forming ranges with each of which being located independently in lines on one surface.
Since the conventional threads being one of liquid crystal fiber products (hereinafter referred to as liquid crystal threads) forms color by using microencapsulized liquid crystals; therefore, in order to obtain enlarged range of color-forming temperatures, plural types of microencapsulized liquid crystals having different color-forming temperatures each are allowed to be used as a mixture. In this case, since the liquid crystals other than one type of liquid crystal under color-formation at a temperature will not develop colors at this temperature, the density of microencapsulized liquid crystals under color-formation become low, resulting in dark coloration being not effective for practical use.